In prior art, it is known that, when a thin film of a material exhibiting a smectic phase is cooled from the liquid phase, the optical appearance of the thin film closely depends upon the rate of cooling; if cooling is slow, the material will orientate itself uniformly and the film will appear perfectly transparent; if, however, the transition from the liquid phase to the smectic phase takes place very rapidly, then in the film domains occur which have different orientations in relation to one another and give rise to strong scattering of the transmitted or reflected light. It is well-known to utilise this effect in order to temporarily record an image on a liquid crystal film presenting a smectic phase. The material, arranged between two glass plates, is maintained at a temperature such that it is in its smectic phase but as close as possible to the transition temperature to the nematic phase; the molecules are uniformly orientated and the film is transparent. A light beam (the terms "light" and "luminous", here as in the remainder of the text, are used in the broadest possible sense to designate electromagnetic radiations in the ultra violet, visible and infra-red parts of the spectrum), generally chosen within the near-infra-red part of the spectrum and intensity modulated, scans the surface of the film. When the energy locally introduced by the beam has been sufficient to produce melting at a point in the film, then, on the occasion of the rapid cooling which follows, a texture forms which diffuses the light whilst the unmelted points remain transparent.
The two problems posed by this method are those of erasing and the production of half-tones.
Erasing can be effected in two ways. The first is to heat the whole film until the liquid phase appears and then to cool it in a controlled manner in order to bring about the formation of an ordered, transparent structure. A second, which enables selective erasing to be carried out, consists in subjecting the film to an alternating transverse electric field having a frequency of the order of 1 kHz, whilst carrying out scanning with the modulated light beam as at the time of recording. The points raised to the melting temperature by the beam, under the orientating influence of the field, return to an ordered smectic phase and therefore become transparent. These two methods of erasing have the drawback that they are slow.
In order to obtain half-tones, it has been suggested to utilize as a thin film, a mixture of two constituents. Then, the fusion of the film does not occur at a well defined temperature, but ranges along a certain temperature gap, for which the material is pasty. The higher is the intensity of the recording beam and consequently the closer the material approaches the highest temperature of this gap, the more pronounced the disorder in the texture obtained after cooling and the correspondant light scattering. Unfortunately, it is difficult in this way to obtain a satisfactory range of half-tones. Moreover, the modulator is an expensive element and its inclusion results in a loss in the power available for recording and therefore in a reduction in the image recording rate.